1. Field of the Invention
The present invention relates to an organic electroluminescence element (EL) using an electroluminescence phenomenon of an organic thin film, and to a process to manufacture an organic electroluminescence element and display unit.
2. Description of the Related Art
An organic electroluminescence element has a structure in which an organic luminescent layer exhibiting at least an electroluminescence phenomenon is sandwiched between an anode electrode and a cathode electrode, and when a voltage is applied between the electrodes, holes and electrons are poured into an organic luminescent layer and by recombining the holes and electrons, the organic luminescent layer becomes a self-luminous element which emits light.
For the purpose of improving luminous efficiency a hole injection layer, a hole transport layer and an electron blocking layer are selected and arranged appropriately between an anode and an organic luminescent layer, and/or a hole blocking layer, an electron transport layer and an electron injection layer, are selected and arranged appropriately between an organic luminescent layer and a cathode. All together an organic luminescent layer, a hole injection layer, a hole transport layer, an electron blocking layer, a hole blocking layer, an electron transport layer, and an electron injection layer or the like, are referred to as a luminescent medium layer.
Each layer of the luminescent medium layer is comprised of an organic material or an inorganic material. An organic material has a low molecular material and a high molecular material.
As examples of using low molecular materials, for example, Copper phthalocyanine (CuPc) is used as a hole injection layer; N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′diamine (TPD) is used as a hole transport layer; Tris(8-quinolinol) aluminium (Alq3) is used as an organic luminescent layer; 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4, oxadiazole (PBD) is used as an electron transport layer; and LiF is used as an electron injection layer.
Generally, each layer of the luminescent medium layer including a low molecular material with a thickness from about 0.1 to 200 nm is deposited mainly by a dry process in a vacuum condition such as a vacuum evaporation method, for example, a resistance heating method or sputtering.
In addition, there are various kinds of low molecular materials and by combining the various materials improvements in luminous efficiency, emission brightness, and life time or the like is expected.
As high molecular materials, for example, the following materials for organic luminescent layers can be used: the material which dissolves low-molecular luminescent coloring matter in polymers such as polystyrene, polymethyl methacrylate and polyvinyl carbazole; macromolecular fluorescent substance such as polyphenylene vinylene derivative (PPV) or poly alkylfluorene derivative (PAF); and polymer phosphor such as rare earth metals.
Generally these high molecular materials dissolve or disperse in a solvent and a film with a thickness from about 1 to 100 nm is formed using a wet process such as coating or printing.
Compared with the use of a dry process in a vacuum condition such as a vacuum evaporation method, the use of a wet process has the following merits: film formation in air is possible, facilities are inexpensive; upsizing is easy; and a film can be formed efficiently in a short time.
In addition, an organic thin film layer using a high molecular material has the following merits: crystallization and cohesion do not occur easily; and because pinholes and foreign matter of the other layers are coated by an organic thin film made of a high molecular material, defects such as short-circuits or dark spots can be prevented.
By contrast, as an inorganic material, alkali metals such as Li, Na, K, Rb, Ce, and Fr, alkaline-earth metals such as Mg, Ca, Sr and Ba, lanthanoid such as La, Ce, Pr, Nd, Sm, Eu, Gd, Db, Dy, Ho, Er, Tm, Yb, Lu, actinoid such as Th, metallic elements such as Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Ar, Nb, Mo, Ru, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Al, Ga, In, Sn, Tl, Pb, and Bi, metalloid elements such as B, Si, Ge, As, Sb, Te, moreover, inorganic compounds such as alloy, oxide, carbide, nitride, boride, sulfide and halide thereof are used for a carrier transport layer.
An inorganic material has better adhesiveness and thermal stability compared with an organic material and improvements in the control of errors in a light emitting phenomenon caused by a leak electrical current, a decrease in the occurrence of non-light emitting areas called dark spots and improvements in emission characteristics and life time of an organic electroluminescence element and display unit are expected. Moreover, because an inorganic material is relatively inexpensive compared with an organic material, when the material is applied to a display unit of large size or large quantity output an inorganic material plays an important role in decreasing costs.
Using the above characteristics, a structure in which an inorganic hole injection layer using an inorganic material is arranged between an organic luminescent layer and an anode which is a hole injecting electrode, is well-known. (Patent Document 1, Patent Document 3, Patent Document 4, Patent Document 5, Cited Document 6, Cited Document 7)
In addition, a structure in which an inorganic electron injection layer using an inorganic material is arranged between an organic luminescent layer and a cathode which is an electron injection electrode is well-known. (Patent Document 2, Patent Document 3, Patent Document 4, Cited Document 7)
Particularly, molybdenum oxide is known as a useful material in which a film is easy to be formed, a hole injecting function from a hole injection electrode is excellent, a stability of transporting holes is high and molybdenum oxide has a good stability and is known as a part of a useful hole transport material and an electron injecting material.
Molybdenum oxide can be divided mainly into molybdic anhydride and molybdenum dioxide. Generally molybdic anhydride is used, because at the time of film formation, transmittance of molybdic anhydride is high and that of molybdenum dioxide is low.
However, since molybdic anhydride is slightly soluble in water, after molybdic anhydride is formed, it becomes easy for solid state properties to change because of a reaction with water. However, molybdenum dioxide and most of the other inorganic compounds are not soluble in water and therefore it is difficult for a change in solid state properties to occur.
In particular, in the case where a luminescent medium layer adjacent to molybdenum oxide is manufactured by a process such as delivery and film formation in which the layer is exposed to air, the film degrades due to degrading factors such as moisture in the air. Therefore, there is a problem of degradation of display characteristics such as luminous efficiency, emission luminance and life time.
That is, in the case where a luminescent medium layer is stacked only by a dry process in a vacuum condition, the effects of degradation factors are few because the number of degradation factors adsorbed on the surface of the molybdic anhydride layer is small. However, in the case where a luminescent medium layer includes a process for forming a film in air, a drastic degradation in display characteristic may be caused.
In addition, in the manufacturing process of forming a film in air such as a wet process, when a solvent of water, alcohol, ketone, carboxylic acid, nitrile and ester is used, molybdic anhydride dissolves in the solvent, the solid state properties and thickness of the film are changed, and in particular, the problem occurs which causes a degradation of luminous efficiency and emission luminance.
Therefore, if molybdenum oxide is used, there was concern that a degradation caused by various degradation factors may occur, and a stable luminescent medium layer could not be formed with every manufacturing process.
The present invention was invented in view of the above circumstances and provides an organic electroluminescent element of high luminous efficiency, high emission brightness and long life time, without defects and protected from the influence of deteriorating factors. In addition, the present invention provides an efficient, inexpensive and stable manufacturing method of an organic electroluminescent element of high luminous efficiency, high emission brightness and long life time, without defects. In addition, the organic electroluminescent element is protected from the influence of deteriorating factors in every process. Furthermore, the present invention provides an inexpensive display of high luminous efficiency, high emission brightness and long life time, without defects and the display is protected from the influence of deteriorating factors.
Patent Document 1: Japanese Patent Laid-Open No.H.11-307259 Official Gazette
Patent Document 2: Japanese Patent Laid-Open No. 2002-367784 Official Gazette
Patent Document 3: Japanese Patent Laid-Open No. H.05-41285 Official Gazette
Patent Document 4: Japanese Patent Laid-Open No. 2000-68065 Official Gazette
Patent Document 5: Japanese Patent Laid-Open No. 2000-215985 Official Gazette
Patent Document 6: Japanese Patent Laid-Open No. 2006-114521 Official Gazette
Patent Document 7: Japanese Patent Laid-Open No. 2006-155978 Official Gazette